Android 8.1.0: microphones work fine for everything except the Android Zoom app

I have a DragonTouch K10 tablet (Android 8.1.0).

Internal, Bluetooth and wired headset audio are all fine with other apps, e.g. AudioLab, where I experience no problem at all with any of the microphones. (AudioLab did need me to opt explicitly to use the Bluetooth microphone the first time.)

But I simply cannot get the Android Zoom app (currently version 6.3.11 (28195)) to pick up audio from any microphone at all, internal, wired, or Bluetooth. Zoom thinks I have a microphone, the microphone permissions setting is in place, but no sound is picked up. Meanwhile the speaker is 100% fine in all cases.

I have uninstalled and reinstalled Zoom on the tablet from the Google Play Store, and most recently tried the app download on the Zoom website, all with no joy.

This site indicates that Zoom fully supports Android 8.1.0.

I have tried the Zoom test (zoom.us/test) which just tells me I have a microphone and then exits (not helpful).

I have run out of things to try, any ideas gratefully received.